Long term bone marrow cultures, monoclonal antibodies and molecular cloning techniques will be used to study cell adhesion molecules (CAMs) that are expressed in bone marrow and other lymphohemopoietic tissues. CAMs may be important for maintaining contact between maturing blood cell progenitors and cells of the microenvironment which produce regulatory cytokines. The movement of cells within marrow and their egress at maturity is probably dependent on CAM modulation and defects in this process could account for abnormal migration of cells in disease. It is also likely that CAMs act as receptors for cell - cell and cell - matrix communication within bone marrow. However, none of these CAM-dependent mechanisms are well understood. Monoclonal antibodies to CD44, VLA4 and VCAM-1 were found to disrupt production of myeloid and/or B lineage lymphoid cells in long term bone marrow cultures and these molecules will receive particular emphasis in the proposed studies. CD44 molecules on some cells, but not others, recognize hyaluronate (HA) as an adhesion ligand. This may result in part from molecular complexity of CD44 protein isoforms and in pan from active regulation of the affinity of CD44 for HA. The cytoplasmic domain of CD44 is important, and the contributions of intracellular and extracellular sequences will be determined by mutagenesis. Certain monoclonal antibodies to CD44 block HA recognition, whereas others markedly enhance this function, perhaps by inducing conformational changes. The position of epitopes recognized by these antibodies will be determined. Additional information will be sought about the distribution and function of these CAMs in vivo and during embryogenesis. The hypothesis will be tested that CAMs mediate transmembrane signaling in bone mar-row cells, resulting in cytokine synthesis and differentiation events. Finally, the same approaches will be expanded to determine what other CAMs are expressed and used by cells in bone marrow. These studies will be informative about. basic mechanisms utilized for blood cell formation and for development of the immune system. Some of the findings may also have implications for diagnosis and treatment of malignancies and other diseases.